A novel application of tau PET in the diagnosis of sporadic inclusion body myositis

Rationale: Sporadic inclusion body myositis (sIBM) is a chronic progressive idiopathic inflammatory myopathy, with characteristic rimmed vacuoles and sarcoplasmic abnormal tau protein deposits. THK5317, an 18 F-labelled positron emission tomography (PET) marker, targets tau protein deposits, which are expressed in the brain of patients with Alzheimer's disease (AD). It is assumed that THK5317 PET/MRI may also depict tau protein in the skeletal muscles of patients with sIBM. Here we introduced a novel application of tau PET in diagnosis of sIBM in a rare case. Patient concerns: We presented a 46-year-old woman who suffered from progressive lower limb weakness for one and a half year. Diagnoses: Needle electromyography showed myogenic damage. Characteristic myopathological changes of sIBM were discovered, and abnormal tau protein deposits were identified by tau immunostaining. Genetic testing ruled out the GNE myopathy, a hereditary distal myopathy with rimmed vacuoles. The patient was finally diagnosed as sIBM. Interventions: We performed [ 18 F] THK5317 PET/MRI on the patient. Outcomes: There were significantly increased tau uptake levels in the quadriceps muscles of sIBM patient. The uptake levels of tau in the quadriceps were significantly higher than that in the posterior group of thigh muscles, which was consistent with the distribution characteristics of involved muscle groups. Lessons: [ 18 F] THK5317 PET can reveal muscular tau deposition in vivo, which provides a new and noninvasive diagnostic method for sIBM and offers the opportunity to monitor the progression of tau pathology along with muscle impairment.